Its the hex representation of the rgba color format.
Rgba color codes are made out of 4 numbers, each in the range of 0 and 255.
R for red
G for green
B for blue
A for alpha (but in tooltips, this value is not used)
If you want a green color for example, you could take R0 G255 B0 A0
On the internet you can find massive lists of nice looking combinations
Hex representation displays each number in a 2 character, base 16 format. 00 being 0 and FF being 255
This is done to always have the color codes in the same length (6 characters for rgb or 8 for rgba)
If you google for something like "rgb to hex" you will find many converters to give you the hex format.
If you type in the color code itself in google, it will even convert it for you by itself.
In warcraft 3, i think that the order is ARGB, so 00FF0000 would be red